How to stop Tip122 from getting hot

Thread Starter

Don_Fila

Joined Nov 26, 2021
171
I visited the shop to get those components that were mentioned of before and they were unavailable in the shop. So I ordered them from different countries. The good news is the circuit is working fine now without getting hot. I have maintained the original circuit but what I did was I have modified the circuit by changing the 24v supply to 17v and the tip122 stop getting hot. The problem is resolved now, but can you please tell me why it's resolve or is it because of the volt that was changed from 24v to 17v
 

Papabravo

Joined Feb 24, 2006
21,159
In the case of the Darlington Regulator, the top trace shows the power dissipation in the TIP122 has dropped from ≈ 3.6 watt down to 1.6 watts which is a huge difference.
The Zener current is still 20 mA after adjusting the value of R1 down a bit, and the transistor still has plenty of base current.

1643565761935.png
 
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dl324

Joined Mar 30, 2015
16,846
As an expert should I leave it how it is now
Don't let labels this website uses sway you. Expert just means I met some arbitrary forum metric, but it's an improvement over "AAC fanatic".

That said, I can't think of any situation where I'd resort to a circuit like this. Well, maybe if I was stranded on an island and those were the only parts I had available..
 

Thread Starter

Don_Fila

Joined Nov 26, 2021
171
Don't let labels this website uses sway you. Expert just means I met some arbitrary forum metric, but it's an improvement over "AAC fanatic".

That said, I can't think of any situation where I'd resort to a circuit like this. Well, maybe if I was stranded on an island and those were the only parts I had available..
 

Thread Starter

Don_Fila

Joined Nov 26, 2021
171
In the case of the Darlington Regulator, the top trace shows the power dissipation in the TIP122 has dropped from ≈ 3.6 watt down to 1.6 watts which is a huge difference.
The Zener current is still 20 mA after adjusting the value of R1 down a bit, and the transistor still has plenty of base current.

View attachment 259207
Ok, so is it going to affect the life of the transistor
 

Papabravo

Joined Feb 24, 2006
21,159
Here's a variation on Papabravo's circuit that uses a programmable TL431 reference instead of a Zener, which has feedback from R1 and R3 to more accurately regulate the output voltage (which is not really needed for this application, of course).
I didn't have the model for the TIP122 Darlington, so I used used two BJTs to emulate it.
I also included the reed relay to control the output from 0V to 12V, which carries less than 2mA when ON.

View attachment 259115
Using the TIP122 model doesn't reveal much in the way of new information

1643572971977.png
 

Papabravo

Joined Feb 24, 2006
21,159
Do I have to limit the base current or what is the best approach?
Unlike the other approaches using a Zener diode with a fixed Zener current , the TL431 looks at the output voltage and adjust the current through it based on the value of the voltage at the control terminal. The control terminal of the TL431A is at the junction of R1 & R3. The value of the voltage at the control terminal that the TL431A wants to "see" is approximately 2.5 volts. R1 & R3 form a voltage divider and the value at the control terminal will be:

\( 12\;\text{Volts}\;\times\;\cfrac{4.99\;\text{K}}{4.99\;\text{K}\;+\;19.1\;\text{K}}\;=\;2.486\;\text{Volts} \)

This tells us that the TL431A(C) is actually going to try to make the output equal a slightly higher voltage. Multiply 2.495 by the reciprocal of the divider ratio and you get

\( 2.495\;\text{Volts}\;\times\;\cfrac{4.99\;\text{K}\;+\;19.1\;\text{K}}{4.99\;\text{K}}\;=\;12.045\;\text{Volts} \)

Taking a look at a fragment of the TL431 datasheet

1643575686841.png

We can see the actual reference voltage will be within a 50 mV range 99.5% of the time. This assumes that the reference voltage is a normally distributed random variable with a mean of 2495 mV and a standard deviation of 25 mV/3 = 8.333 mV. That is what I understand the MIN and the MAX on a datasheet to imply. You can spend extra money on resistors with more precision, but 1% really should be good enough.
 

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sghioto

Joined Dec 31, 2017
5,380
I have maintained the original circuit but what I did was I have modified the circuit by changing the 24v supply to 17v and the tip122 stop getting hot.
The circuit you have built with the parts you have on hand is fine for this application. When you reduced the input voltage to 17 volts you also reduced the voltage across the TIP122 from 12 to 5 volts. Power = (current X voltage) so for a given current which is the current drawn by the 2 relays X 5 volts is appx 60% less then at 12 volts which also means the TIP has 60% less dissipation in the form of heat. One improvement to the circuit is to add a 100uf capacitor from the base of the TIP122 to ground.
 

Thread Starter

Don_Fila

Joined Nov 26, 2021
171
The circuit you have built with the parts you have on hand is fine for this application. When you reduced the input voltage to 17 volts you also reduced the voltage across the TIP122 from 12 to 5 volts. Power = (current X voltage) so for a given current which is the current drawn by the 2 relays X 5 volts is appx 60% less then at 12 volts which also means the TIP has 60% less dissipation in the form of heat. One improvement to the circuit is to add a 100uf capacitor from the base of the TIP122 to ground.
Ok but what will be the volt of the capacitor ?
 
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